Digital cross-connect communications network devices, such as the 1631 SX manufactured by Alcatel Network Systems, Inc., are designed to make connections between input gates on input switches and output gates on output switches. Known designs for such cross-connect devices make use of matrices to connect the input gates to the output gates. To limit the physical space and manufacturing costs of the matrices, designers seek to minimize the number of cross-connects in the matrix. This objective becomes more complicated as market demands for communications services increase. In particular, as network devices address larger markets, the devices must service more and more input and output signals. These factors exacerbate the space and cost limitation problems even more for communications matrix devices.
One matrix configuration that uses a minimal number of matrix cross-connects for a given number of input gates and output gates and that provides a potentially optimal solution is known as a "rearrangeable" matrix. For the rearrangeable matrix, the number of center stage switches must equal or exceed the number of input gates on each input switch of the matrix. In a rearrangeable matrix, there exists a set of conditions such that, although the device does not use all input gates and all output gates, an attempt to use an idle input gate and an output gate is prohibited because existing connections block the signal flow through the matrix. This may happen, for example, if existing connections already occupy at least one link in every possible path between the input and output gates in question.
In a rearrangeable network, it is always possible to unblock a flow path from an idle input gate to an idle output gate by moving existing connections in the network The term "rearrangeable," therefore, describes the property that for a given state of a network and any given idle pair of input and output gates, the existing connections of the matrix may be reassigned to new paths, if necessary, to connect the idle pair.
Existing methods and systems for connecting inputs to outputs in rearrangeable matrices generally use a standard rearrangement technique that determines which cross-connects of the matrix to rearrange to permit a signal to flow. N. C. Paull in "Reswitching of Connection Networks," The Bell System Technical Journal, May, 1962, pp. 833-856, describes this known method for unblocking a rearrangeable matrix. This method (hereinafter referred to as Paull's Method) suffers from a major limitation. Paull's Method requires breaking some of the crossconnects and making some other of the cross-connects to rearrange matrix. This procedure takes time and results in undesirable service delays or interruptions during matrix rearrangement.
It is an object of the present invention, therefore, to provide a method and system that connect input gates to output gates while permitting both rearrangement of the matrix and uninterrupted signal flow through the matrix during the rearrangement. The present invention achieves this object with a minimal amount of additional circuitry and avoids the service delays and interruptions of known rearrangement methods and systems.
It is also an object of the present invention to provide a method and system for rearranging a rearrangeably blocked communications matrix and maintaining hitless connections of previously connected signals by forming an immediate connection configuration of a predetermined input gate to predetermined output gate through a dedicated center stage switch, and then using a predetermined center stage switch other than the dedicated center stage switch to establish a parallel connection configuration through a dedicated center stage switch and a predetermined center stage switch from the predetermined input gate to the predetermined output gate and then rearranging connections through the communications matrix including the dedicated center stage switch to establish parallel connections that maintain signal flow through the communications matrix and that includes the dedicated center stage switch and, once the parallel connection are established, to break connections through dedicated center stage switch. The result is that by the invention using the parallel connection from the input gates (herein called a head-end-bridge) and to the output gates (herein called a receive-end-switch) through the dedicated center stage switch, hitless rearrangements of communications, matrix connection are possible.